Field of the Invention
The present invention relates generally to optical elements and more specifically to infrared transmitting graded index optical elements.
Description of the Prior Art
Graded index (GRIN) optics focus light through the use of an index profile in the lens instead of refracting the light of each surface as is the case in conventional lenses. For example, the flat ends of a GRIN lens enable more efficient coupling to fiber optics. GRIN lenses can reduce aberrations, and replace multiple lens elements in a lens system thereby permitting compact, lightweight lenses to replace bulky heavy lenses in existing systems.
Currently, GRIN optics are made from monovalent ion-doped silicate glasses or polymers. In both cases, there is no light transmission in the infrared (IR) beyond a wavelength of about two microns, an important wavelength region for many applications, such as sensing. The method used to make the silicate GRIN optics relies on ionic diffusion from the outside surface to the inside to create the index profile. This is a slow process since the diffusion lengths are large. For example, it can take as long as 200 hours to ion exchange a 2 mm thick silicate glass to make a GRIN lens. Additionally, diffusion limits the complexity of the profiles that can be made using traditional ion exchange. The polymer systems suffer from a small index difference (˜0.1). This limits the usefulness of polymer GRIN lenses.
A microlayering/molding process has been used to fabricate plano-convex GRIN lenses from alternating layers of two immiscible polymers in a structure that mimics the ocular lens of fish (Beadie et al., “Optical properties of a bio-inspired gradient refractive index polymer lens,” Opt. Exp., 16 (15), 11540-11547 (2008), the entire contents of which is incorporated herein by reference). These lenses are not suitable for IR light due to the high IR absorption of polymers. IR light is defined as a wavelength beyond about 2 μm. For example in the 2-5 μm region, the 8-12 μm region, and the 2-12 μm region. Further, those skilled in the art, will quickly see that the process is not applicable to IR materials since such materials (IR crystals and ceramics) do not exhibit the plasticity needed for microlayer extrusion.